2 Answers
2

Moving Provokes: If an enemy leaves a square adjacent to you, you can make an opportunity attack against that enemy. However, you can’t make one if the enemy shifts or teleports or is forced to move away by a pull, a push, or a slide.

Exception does not include burrowing. But more importantly:

Interrupts Target’s Action: An opportunity action takes place before the target finishes its action. After the opportunity attack, the creature resumes its action.

The interrupt (not a reaction) is key. So before the burrowing happens, the Opportunity Attack happens.

A creature that has a burrow speed can move through loose earth at a specified speed, and the creature can move through solid stone at half that speed. The creature can’t shift or charge while burrowing.

The reason to prohibit shifting during burrowing is to explicitly allow for
Opportunity Attacks.

\$\begingroup\$But what happens if the creature is already in the ground?\$\endgroup\$
– SteveCJun 4 '12 at 16:01

1

\$\begingroup\$@SteveC Assuming the attacker is on the surface and has no special abilities to the contrary, a burrowed creature would not be in "threatened square", since there is blocking terrain between them.\$\endgroup\$
– F. Randall FarmerJun 4 '12 at 21:05

\$\begingroup\$This certainly applies to a creature that is not leaving an adjacent square, but rather tunnelling under you, but since OA's interrupt (and in the case of fighters can prevent) movement away, the OA should happen before the target is burrowing and therefore before LOS is lost, in the "burrow away" scenario in the question.\$\endgroup\$
– Simon WithersJun 4 '12 at 4:25

\$\begingroup\$@SimonWithers The notes in the On-line Compendium suggest that a creature can have bother a Burrow speed, and the Tunneling effect ... yeap, just checked and e.g. Lesser Earth Elemental only has Burrow whereas Hive Worker has Burrow (Tunneling) as one its Speeds ... in the 3D 4E burrowing underneath you is still an adjacent square\$\endgroup\$
– SteveCJun 4 '12 at 16:08

\$\begingroup\$@SteveC Adjacent spaces are not always "threatened" (LOS/LOE) and subject to OA, though I can see how this case is a little persnickety. Thin walls are still walls. I hate -1 votes too, btw.\$\endgroup\$
– F. Randall FarmerJun 4 '12 at 18:15